U.S. Pat. No. 3,893,869 discloses a cleaning system wherein very high frequency energy is employed to agitate a cleaning solution to loosen particles on the surfaces of semiconductor substrates or wafers. Maximum cleanliness for such items is desired in order to improve the yield of acceptable semiconductor chips made from such wafers. This cleaning system has become known as megasonic cleaning, in contrast to ultrasonic cleaning in view of the high frequency energy employed. Megasonic cleaning has a number of known advantages over ultrasonic cleaning.
One of the major improvements that help make the product a commercial reality concerns the design of the transducer array which converts electrical energy into sound waves for agitating the cleaning liquid. The transducer array is perhaps the most critical component of the megasonic cleaning system. A transducer array which was marketed for a number of years by Verteq, Inc., of Santa Ana, Calif., utilized a transducer array mounted in the bottom of a process tank. The array included a strong, rigid frame with a very thin foil layer of tantalum, which is a ductile acid-resisting metallic element, spread over the upper surface of the frame. Ceramic transducers were positioned within a space in the plastic frame and bonded to the lower side of the tantalum layer. The frame was then attached to the bottom wall of the container with the tantalum being exposed to the interior of the container.
In later improvements, the tantalum foil was replaced by quartz as a transmitter, which has provided a number of advantages. Such arrangements are shown in U.S. Pat. Nos. 4,804,007, 4,998,549 and 5,037,481. The use of quartz as the material that transmits the megasonic energy into the tank has also led to the mounting of the megasonic energy transducer directly to the wall of a quartz tank. There are, however, a number of drawbacks and complexities with that approach. In initially installing a transducer, there is always the risk of damage to the tank, and tanks are relatively expensive. Further, if the end user of the apparatus does not have the capability for servicing or installing the transducer array, it is necessary to ship the entire tank to the entity that will provide such service. This means that the tank is out of commission for the time required to ship and service the unit, and this adds to the expense of the process. Further, if the portion of the container which is to serve as the transducer is curved rather than flat, the initial expense and the expense of delay are increased.
Because of these complexities, another approach that has been used is to form the transducer array as a separate unit mounted on a frame and then to attach the frame to a tank wall with the transmitter extending through a hole in the tank wall. An advantage of this approach is that the transducer array can be completely assembled and then mounted as a unit on the tank, and hence can be serviced and replaced more simply than if the entire tank has to be shipped or otherwise handled. On the other hand, there are also drawbacks to this technique. A primary drawback is that fasteners and seals are required to hold the various components together and to prevent leakage of fluids. This normally means that seals or other connecting components are exposed to the cleaning solutions to be placed in the tanks, and this in turn introduces compatibility and cleanliness issues. Also, there are connection difficulties in that fasteners cannot normally be threaded into quartz with sufficient strength or durability. Thus, typically the tank is made of plastic rather than quartz. While there are some excellent plastics available, quartz is preferable from a cleanliness standpoint. As cleanliness requirements become more stringent, the demand for quartz tanks has increased.
In view of the foregoing, a need exists for an improved system for mounting a transducer array to a quartz tank. Desirably, the improved technique will overcome the shortcomings of prior arrangements, but will maintain the advantages of the known arrangements.